Engine Basics: Detonation and Preignition
Part 1 of 3
By Allen W. Cline
Reprinted from Issue 54 of CONTACT! Magazine, published in January 2000
This IO-550-N installed in the 2011 AirVenture Bronze Lindy award-winning Lancair Legacy, built by Jay Sabot, is an outstanding example of how beautiful an aircraft engine can be. But understanding what's going on inside and being able to operate it within its design limits is far more important than eye candy.
All high-output engines are prone to destructive tendencies as a result of overboost, misfueling, mistuning, and inadequate cooling. The engine community pushes ever nearer to the limits of power output. As it often learns, cylinder chamber combustion processes can quickly evolve into engine failure. This article defines two types of engine failures, detonation and preignition, each as insidious in nature to users as they're hard to recognize and detect. This discussion is intended only as a primer about these combustion processes since whole books have been devoted to the subject.
First, let us review normal combustion. It's the burning of a fuel and air mixture charge in the combustion chamber. It should burn in a steady, even fashion across the chamber, originating at the spark plug and progressing across the chamber in a three-dimensional fashion. Similar to a pebble in a glass-smooth pond with the ripples spreading out, the flame front should progress in an orderly fashion.
The burn moves all the way across the chamber and quenches (cools) against the walls and the piston crown. The burn should be complete with no remaining fuel-air mixture. Note that the mixture doesn't "explode" but burns in an orderly fashion.
There's another factor that engineers look for to quantify combustion. It's called "location of peak pressure" (LPP) which is measured by an in-cylinder pressure transducer. Ideally, the LPP should occur at 14 degrees after top dead center. Depending on the chamber design and the burn rate, if one would initiate the spark at its optimum timing (20 degrees before top dead center [BTDC] for example), the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees after top dead center (ATDC).
LPP is a mechanical factor just as an engine is a mechanical device. The piston can only go up and down so fast. If you leak the pressure too soon or too late in the cycle, you won’t have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine.
I introduce LPP now to illustrate the idea that there's a characteristic pressure buildup (compression and combustion) and decay (piston downward movement and exhaust valve opening) during the combustion process that can be considered "normal" if it's smooth, controlled and its peak occurs at 14 degrees ATDC.
Our enlarged definition of normal combustion now says that the charge/bum is initiated with the spark plug, a nice even burn moves across the chamber, combustion is completed, and peak pressure occurs at 14 degrees ATDC.
Confusion and a lot of questions exist as to detonation and preignition. Sometimes you hear mistaken terms like "predetonation." Detonation is one phenomenon that is abnormal combustion. Preignition is another phenomenon that is abnormal combustion. The two, as we will talk about, are somewhat related but are two distinctly different phenomenon and can induce distinctly different failure modes.
Detonation: Detonation is the spontaneous combustion of the end gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.
Preignition: Preignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event, it's classified as preignition. The two are completely different and abnormal phenomenon.
Unburned end gas, under increasing pressure and heat (from the normal progressive burning process and hot combustion chamber metals), spontaneously combusts, ignited solely by the intense heat and pressure. The remaining fuel in the end gas simply lacks sufficient octane rating to withstand this combination of heat and pressure.
Detonation causes a very high, very sharp pressure spike in the combustion chamber, but it's of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, then all of a sudden you would see a very sharp spike when the detonation occurred. That spike always occurs after the spark plug fires.
The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about 6400 hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of detonation is commonly called spark knock. The sound changes only slightly between iron and aluminum.
This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to 6400 hertz, and they will pick up that spark knock. Incidentally, the knocking or pinging sound isn't the result of "two flame fronts meeting" as is often stated. Although this clash does generate a spike, the noise you sense comes from the vibration of the engine structure reacting to the pressure spike.
One thing to understand is that detonation isn't necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. If you've driven a car that has a lot of spark advance on the freeway, you'll hear it pinging. It can run that way for thousands and thousands of miles.
Detonation isn't necessarily destructive. It's not an optimum situation, but it isn't a guaranteed instant failure. The higher the specific output (hp/inch3) of the engine, the greater the sensitivity to detonation. An engine that is making 0.5 hp/inch3 or less can sustain moderate levels of detonation without any damage. But with an engine that is making 1.5 hp/inch3, if it detonates, it will probably be damaged fairly quickly—within minutes.
Detonation causes three types of failure:
- mechanical damage (broken ring lands)
- abrasion (pitting of the piston crown)
- overheating (scuffed piston skirts due to excess heat input or high coolant temperatures).
The high impact nature of the spike can cause fractures. It can break the spark plug electrodes, the porcelain around the plug; cause a clean fracture of the ring land; and can actually cause fracture of valves—intake or exhaust. The piston ring land, either top or second depending on the piston design, is susceptible to fracture type failures. If I were to look at a piston with a second broken ring land, my immediate suspicion would be detonation.
Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of look if detonation occurs. It is a Swiss-cheesy look on a microscopic basis. The detonation, the mechanical pounding, actually mechanically erodes or fatigues material out of the piston.
You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because if you think about it, you would ignite the flame front at the plug. It would travel across the chamber before it got to the farthest reaches of the chamber where the end gas spontaneously combusted. That's where you will see the effects of the detonation; you might see it at the hottest part of the chamber in some engines, possibly by the exhaust valves. In that case, the end gas was heated to detonation by the residual heat in the valve.
In a four-valve engine with a pent roof chamber with a spark plug in the center, the chamber is fairly uniform in distance around the spark plug. But one may still see detonation by the exhaust valves because that area is usually the hottest part of the chamber. Where the end gas is going to be hottest is where the damage, if any, will occur.
Because this pressure spike is very severe and of very short duration, it can actually shock the boundary layer of gas that surrounds the piston. Combustion temperatures exceed 1800 degrees. If you subjected an aluminum piston to that temperature, it would just melt. The reason it doesn't melt is because of thermal inertia and a boundary layer of a few molecules thick next to the piston top.
This thin layer isolates the flame and causes it to be quenched as the flame approaches this relatively cold material. That combination of actions normally protects the piston and chamber from absorbing that much heat. However, under extreme conditions the shockwave from the detonation spike can cause that boundary layer to break down which then lets a lot of heat transfer into those surfaces.
Engines that are detonating will tend to overheat because the boundary layer of gas is interrupted against the cylinder head and heat is transferred from the combustion chamber into the cylinder head and the coolant. So it starts to overheat. The more it overheats, the hotter the engine, the hotter the end gas, the more it wants to detonate, the more it wants to overheat. It's a snowball effect. That’s why an overheating engine wants to detonate, and that's why engine detonation tends to cause overheating.
Many times you'll see a piston that is scuffed at the "four corners." If you look at the bottom side of a piston, you can see the piston pin boss. If you look across each pin boss, you'll find it's solid aluminum with no flexibility. It expands directly into the cylinder wall. However, the skirt of a piston is relatively flexible. If it gets hot, it can deflect.
The crown of the piston is actually slightly smaller in diameter on purpose so it doesn't contact the cylinder walls. However, if the piston soaks up a lot of heat, because of detonation for instance, the piston expands and drives the piston structure into the cylinder wall, causing it to scuff in four places directly across each boss. It's another dead giveaway of detonation. Many times detonation damage is just limited to this outcome.
Some engines, such as liquid-cooled two-stroke engines found in snowmobiles, watercraft, and motorcycles, have a very common detonation failure mode. What typically happens is that when detonation occurs, the piston expands excessively, scuffs in the bore along those four spots, and wipes material into the ring grooves. The rings seize so that they can't conform to the cylinder walls. Engine compression is lost, and the engine either stops running or you start getting blow-by past the rings. That torches out an area. Then the engine quits.
In the shop someone looks at the melted result and says, "Preignition damage." No, it's detonation damage. Detonation caused the piston to scuff, and this snowballed into loss of compression and hot gas escaping by the rings that caused the melting. Once again, detonation is a source of confusion; it's very difficult sometimes to pin down what happened, but in terms of damage caused by detonation, this is another typical sign.
While some of these examples may seem rather tedious, I mention them because a "scuffed piston" is often blamed on other factors, and detonation as the problem is overlooked. A scuffed piston may be an indicator of a much more serious problem which may manifest itself the next time with more serious results.
In the same vein, an engine running at full throttle may be happy due to a rich wide open throttle (WOT) air/fuel ratio. Throttling back to part throttle, the mixture may be leaner and detonation may now occur. Bingo, the piston overheats and scuffs; the engine fails. But the postmortem doesn't consider detonation because the failure didn't happen at WOT.
I want to reinforce the fact that the detonation pressure spike is very brief and that it occurs after the spark plug normally fires. In most cases, that will be well after ATDC, when the piston is moving down. You have high pressure in the chamber anyway with the burn. The pressure is pushing the piston like it's supposed to, and superimposed on that you get a brief spike that rings the engine.
In the next installment, which you'll find in the February 2012 issue of Experimenter, we'll discuss the causes of detonation, how chamber designs affect detonation, as well as detonation indicators. In March, look forward to reading more about preignition, and we'll be sure to muddy the water with a little thing called detonation-induced preignition.